The fifth generation (5G) of mobile telecommunications and wireless technology is not yet fully defined but in an advanced draft stage within 3rd Generation Partnership Project (3GPP). It includes work on 5G New Radio (NR) Access Technology. Long term evolution (LTE) terminology is used in this disclosure in a forward-looking sense, to include equivalent 5G entities or functionalities although a different term is specified in 5G. A general description of the agreements on the physical layer aspects of 5G NR Access Technology so far is contained in 3GPP Technical Report 38.802 v1.2.0 (2017-02). Final specifications may be published inter alia in the future 3GPP TS 38.2** series.
FIG. 1 schematically illustrates a wireless communication network, where a User Equipment UE1 is wirelessly connectable to a Base Station BS 2. The BS 2 is connected to a core network CN 3. In an NR access network, the BS may be referred to as a gNB, and the corresponding terminology for an LTE access network is an eNB. The BS 2 serves the UE1 located within the BS's geographical area of service, called a cell.
Initial Access and Synchronization in Cellular Systems
When a wireless device (or UE) first accesses a wireless communication system, it must synchronize to the system. The synchronization is required for the UE to know when the network will transmit various signals such as broadcast of system information (SI). The UE must also synchronize to the system to understand when it should transmit uplink signals, such as the random access signals transmitted during initial access.
A wireless communication system uses different time units to keep track of time. In systems using orthogonal frequency division multiplexing (OFDM), the term OFDM symbol is used for the smallest time unit. A number of symbols may form slots, a number of slots may form subframes, and a number of subframes may form radio frames. System information and paging information are typically distributed on a time scale where a radio frame is a relevant time unit. In many cellular system standards, a radio frame is 10 milliseconds in length.
In LTE, there are two synchronization signals: Primary synchronization signal (PSS) and Secondary synchronization signal (SSS). To perform initial access, the UE must obtain at least symbol and frame synchronization. To obtain symbol synchronization, the UE searches for a special synchronization sequence, which corresponds to the PSS. The PSS is typically one symbol long. By finding that sequence, the UE can establish symbol timing. The UE may also use the received PSS to determine frame synchronization. For that to be possible, every PSS must be transmitted with a fixed timing relation to the frame start. When the UE has found the PSS, it can also read an identifier of the current cell, and very basic system information, called the master information block (MIB). The PSS and SSS are thus used to indicate the physical-layer cell identity (PCI) to a UE, besides the functionality to provide the synchronization.
In NR, the concepts of PSS and SSS are re-used to provide the initial synchronization and are referred to as NR-PSS and NR-SSS. NR-PSS is defined for initial symbol boundary synchronization to the NR cell. NR-SSS is defined for detection of NR cell identity (cell ID) or at least part of NR cell ID.
In NR, a broadcast channel referred to as NR Physical Broadcast Channel (NR-PBCH) is defined. NR-PBCH is a non-scheduled broadcast channel carrying a part of minimum system information with fixed payload size and a periodicity predefined in the specification depending on carrier frequency range. NR-PBCH contents shall include at least part of the system frame number (SFN), and a Cyclic Redundancy Check (CRC). The following is a list of options to what the NR-PBCH may carry in terms of system information:                Option 1: NR-PBCH carries a part of essential system information for initial access including information necessary for UE to receive a channel carrying remaining essential system information;        Option 2: NR-PBCH carries minimum information necessary for UE to perform initial UL transmission in addition to the information in Option 1 allowing for initial access; and        Option 3: NR-PBCH carries all essential system information for initial access.        
In NR, it will be possible to transmit the NR-PSS using beamforming. The NR-PSS will be transmitted in different beams at different time instants. The beams over which the NR-PSS is transmitted are chosen so that a UE at any position in the cell can receive at least one NR-PSS transmission. Sometimes, the term beam sweep is used for this procedure. To support beam sweeping of the NR-PSS, more than one NR-PSS must be transmitted in each frame, otherwise, the synchronization delay will be too long. This means that NR-PSSs transmitted in different beams will have different offsets relative to the frame start, which in turn means that the UE cannot derive the frame start only from the time when it receives the NR-PSS. Some additional information is required.
To support beam sweeping with massive Multiple Input Multiple Output (MIMO), a new concept of SS block has been defined to include some basic signals and broadcast system information. NR-PSS, NR-SSS and/or NR-PBCH can be transmitted within an SS block. However, multiplexing other signals within an SS block is not precluded. A UE shall be able to identify an OFDM symbol index, a slot index in a radio frame, and a radio frame number from an SS block.
In the 3GPP agreements for NR, a basic structure for the synchronization signals and channels has been defined. FIG. 2b shows a schematic diagram of the basic structure for the synchronization signals transmission. One or multiple SS block(s) compose an SS burst. One or multiple SS burst(s) further compose an SS burst set, where the number of SS bursts within a SS burst set is finite. The number of SS block(s) composing one SS burst set is L in the example illustrated in FIG. 2b, where L is a positive integer. From physical layer specification perspective, at least one periodicity of the SS burst set is supported. From a UE perspective, an SS burst set transmission is periodic, and a UE may assume that a given SS block is repeated with a SS burst set periodicity.
3GPP has decided that there may be up to 64 SS blocks in an SS burst set. The minimum periodicity for SS block sets is 5 ms, and a radio frame is 10 ms. Thus, the number of SS blocks in a radio frame may be up to 128.